Biomedical Engineering Reference
In-Depth Information
10
Sensorless Adaptive
Optics for Microscopy
10.4. Sensorless.Adaptive.Optics.Using.Modal.
. Wavefront.Sensing............................................................................. 181
10.9. An.Empirical.Approach.for.the.Derivation.of.
Martin J. Booth
University of Oxford
Alexander Jesacher
Innsbruck Medical
University
10.1 introduction
he.imaging.properties.of.a.microscope.sufer.from.the.presence.of.aberrations,.and.optimum.perfor-
mance.is.obtained.only.when.aberrations.are.zero..As.aberrations.are.introduced.by.imperfections.in.
the.optical.system.or.by.the.inhomogeneous.refractive.index.distribution.of.the.specimen,.most.micro-
scope.systems.are.in.some.way.afected..To.ensure.optimum.image.quality,.optical.systems.are.designed.
so.that.the.overall.aberration.is.set.below.an.acceptable.tolerance..However,.if.the.system.is.used.outside.
of. its. design. speciications,. including. when. the. specimen. refractive. index. difers. from. the. objective.
immersion.medium,.then.aberrations.can.be.signiicant..Correction.of.these.aberrations.through.adap-
tive.optics.(AO).is.essential.if.optimum.imaging.performance.is.to.be.ensured.
In.AO,.an.adaptive.element,.such.as.a.deformable.mirror.(DM),.is.used.to.introduce.additional.wave-
front.distortions.that.cancel.out.other.aberrations.in.the.system..Traditional.AO.systems,.such.as.those.
developed. for. astronomical. telescopes,. also. use. a. wavefront. sensor. to. measure. aberrations.. Common.
methods.for.direct.wavefront.sensing.are.the.Shack-Hartmann.sensor.(Hardy.1998).or.devices.based.on.
interferometry.(Hariharan.2003)..An.advantage.of.these.sensors.is.their.speed,.which.makes.them.the.
method.of.choice.for.astronomical.or.free-space.optical.communication.applications,.where.corrections.
have.to.be.performed.at.the.fast.timescale.given.by.air.turbulence.
Although.AO.techniques.have.been.applied.to.microscopy,.most.of.these.systems.have.not.used.wave-
front.sensors.but.have.instead.relied.on.indirect.aberration.measurements.(Booth.2007a).(
Figure.10.1
)
..
One.reason.for.this.is.that,.for.many.microscopy.applications,.it.is.not.straightforward.to.apply.direct.
wavefront.sensing.because.of.the.three-dimensional.nature.of.the.specimen..he.sensor.would.ideally.
detect.light.emanating.from.the.focal.region,.but.light.is.instead.emitted.from.a.larger,.three-dimensional.
177
